I. Field of the Invention
The present invention relates to radio frequency (RF) amplifiers. More particularly, the present invention relates to a novel and improved dual-mode RF amplifier that exhibits both high efficiency and high linearity.
II. Description of the Related Art
As is known in the art of amplifier design, high linearity and high efficiency are generally mutually exclusive design considerations. That is to say that when one is designing a particular transistor-based amplifier, one must usually make a tradeoff between the high linearity and high efficiency. The difference between high linearity and high efficiency is manifested by saturation characteristics which are determined by the load impedance in relation to the current capability and breakdown voltage of the amplifier. Thus, a designer who wishes to design a highly linear amplifier will generally choose a relatively low load impedance for a given supply voltage. Highly linear amplifiers maintain the integrity of the input signal envelope at the expense of higher average power dissipation. This high average power dissipation which results from overlap of current and voltage in the transistor over time is particularly undesirable in a battery-powered portable transmitter because it reduces the battery life, and thus the transmit time of the portable transmitter between battery charges.
Conversely, a designer who wishes to design a highly efficient amplifier will generally choose a relatively higher load impedance for the same supply voltage. Highly efficient amplifiers maintain a lower average power dissipation at the expense of "clipping" of the input signal at high input amplitudes due to premature saturation of the amplifier. Although clipping the input signal gives rise to high efficiency and longer battery life because the device's power dissipation (and thus the instantaneous voltage/current overlap and power dissipation) is minimized during saturation, it results in distortion of the input signal envelope, and consequent generation of in-band spectral sidelobes of information. Furthermore, clipping generates higher-order harmonics that may be spread outside of the allowed operating bandwidth of the transmitter, causing interference to other RF devices transmitting or receiving on other frequencies.
In the field of wireless telecommunications, such as in various cellular, Personal Communication Services (PCS), and wireless local loop (WLL) communication systems, many different communication standards exist for these wireless communication systems. For example, Code-Division Multiple Access (CDMA) digital communications may be governed in the United States by either Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) Interim Standard IS-95 for cellular bands, or ANSI J-STD-008 for PCS bands. Additionally, Time-Division Multiple Access (TDMA) digital communications may be governed by the TIA/EIA IS-54 or by the European standard Global System for Mobile Communications (GSM). Finally, analog FM-based communications may be governed by the Advanced Mobile Phone System (AMPS) standard, or one of its improvement standards such as N-AMPS.
For each of these communication system standards, a long-felt need exists for an amplifier for a wireless communication device which exhibits the high linearity needed for signal integrity, as well as the high efficiency needed for longer operating time. This is particularly true for dual-mode communication devices that can operate according to two different standards (such as CDMA/AMPS), because each of the standards may have different linearity requirements. For example, the linearity requirements in a CDMA communication device are more stringent than those of an AMPS communication device which has no in-band linearity requirement. Thus a dual-mode CDMA/AMPS communication device would benefit greatly from being able to take advantage of a high linearity amplifier while operating in a CDMA mode, while still being able to operate with high efficiency while in the AMPS mode where there are no in-band linearity requirements.
Although there have been various attempts to create a highly efficient amplifier that is also highly linear, these attempts contain inherent problems which limit their effectiveness. For example, Doherty-type amplifiers are well known in the art as being highly efficient and also highly linear. A Doherty-type amplifier modulates the load impedance in response to the envelope of the input signal by using two amplifiers in parallel, and the output of one of the amplifiers in series with a quarter-wavelength phase shifter. An example of such an amplifier is illustrated in U.S. Pat. No. 5,568,086 to Schuss et al., entitled "LINEAR POWER AMPLIFIER FOR HIGH EFFICIENCY MULTI-CARRIER PERFORMANCE." However, a significant drawback to the Doherty-type design of Schuss et al. is that a quarter-wavelength phase shifter may be difficult and costly to realize at certain frequencies in the mobile telephone environment, such as the 850 MHz cellular band. Additionally, Doherty-type amplifiers are narrowband "tuned" amplifiers that operate best around a single frequency and are ill-suited for broadband use mobile telephone environment.
Another example solution is illustrated in U.S. Pat. No. 5,175,871 to Kunkel, entitled "POWER AMPLIFIER FOR A CELLULAR TELEPHONE." The amplifier of Kunkel uses a one non-linear amplifier stage which may be used when non-linear behavior is desired, and one linear amplifier stage which may be switched in when linear power amplification is desired. However, a significant drawback of Kunkel is the switch loss which reduces analog mode efficiency. An additional drawback is the increased expense of providing two separate amplifiers, each with its own design characteristics.
Thus, there is a need for an amplifier that is both highly efficient and highly linear which avoids the problems of the prior art.